Three's a Crowd...or is it?

Due to recent events that have taken place, I am left to re-visit this age-old question, to which there is no correct answer. Is it ok for people to have best friends of the opposite sex while in a relationship?And if we wanna dig even deeper, is it ok for a person in a relationship to have a close friend who once used to be a partner?
I have mixed feelings about this answer that I can't even explain myself. Perhaps, it is because I've been on both ends of the spectrum. I've had best friends of the male species for most of my life. Of which, there has never been ANY romantic interest. And I've had their significant others to become uncomfortable with our bond. I get it. No one wants to feel like there is a person of the opposite sex that is closer to their partner than they are. It can sort of feel like an imposition by the 3rd party. But do we call that insecurity, or fighting temptation?! Ultimately, if someone wants to cheat, they will. Regardless of whereabouts, check-ins, compromise or what have you. But at the same time, I have also known people who have been extremely close to me tell me that they don't see me as a "sister-like" figure anymore. So, by having that friend...are we just flirting with temptation? Entertaining enticement, or arousing "what could be// and or what used to be?"

One of my best friends in the whole wide world was a guy. He had been my friend since I was 9 years old, and could tell me more about myself than any other person...including me sometimes. But by the time I was in college, we did become romantically involved. Granted, it wasn't for a long time, but we had to see if our love for one another surpassed that of friendship. It sort of did, but it was one of those "I know WAY too much about you to want to be with you" kind of things. At least on my end, but I did love him unconditionally. After our little period, I had no desire whatsoever to be with him, again, I knew too much about him, and we (though I hate sounding so incestuous by saying it) found that we were more like brother and sister. In any case, in our adulthood, both of us were to become someone else's spouse. And his, did NOT approve of our friendship. Though it hurt me dearly because he had been such an integral part of my life, I humbly respected her wishes. She wasn't looking at it like, "oh, that's been his bff since they were knee-high to a duck." She's saw me as someone that he'd been with. And even though he meant nothing to me in a romantic way, it was as simple as keeping his family together. And just like that, an almost 20 yr. friendship was over. I was devastated not to be able to see him exchange nuptials. I can't blame her, though. When I'd gotten married, my (at the time) husband wanted to invite his ex to the wedding. And I'm like "why?" It may be a little awkward to have her sitting there thinking of all the things they used to do while he was at that point doing those things with me. So, given these observations and experiences...what do you think? Is three a crowd or are you comfortable with it?